Could anyone tell me where B31 is in Changi T2 & Row 4/Door 3 Gate B7 is in T3 please.I have tried to look myself, but found the layout very confusing

Firstly there are no B-gates in T2, only E and F gates. Perhaps you're referring to E31. This gate is the second gate on the right "horn" after Immigration.

For T3, Door 3 means the 3rd door at the departure drop-off driveway and Row 4 is the check-in row that's just inside Door 3. Gate B7 is somewhere in the middle of the right "arm" after you go though Immigration.

Appreciate if someone can help give better descriptions between a "horn" and "arm" boarding sections.